Measuring head systems for coordinate measuring machines comprising a mechanical scanning element have been used for many years for exact surveying of the surface of a measured object. A measuring head system of the generic type, also referred to as scanning head, for a corresponding coordinate measuring device is disclosed, for example, in EP 0 373 644 A1.
For example, the object shape of mechanically produced workpieces is checked therewith for quality control. For the measuring process, the measuring head of the coordinate measuring device is brought towards the measured object by means of a displacement frame until a scanning element mounted so as to be displaceable relative to a measuring head base of the measuring head, in particular a stylus, touches a desired measuring point on the measured object. The spatial coordinates of the scanned measuring point can then be determined from the position of measuring head and the position of the scanning element relative to the measuring head base.
Various methods are known for determining the position of the stylus relative to the measuring head base, in other words the deflection of the stylus during scanning of the measured object.
In the case of the measuring head system from EP 0 373 644 A1 mentioned at the outset, an optical detector is used for this purpose. This comprises two light transmitters, which produce two light beams converging diagonally towards one another, and a two-dimensional PSD (position sensitive device) as a light receiver. When the stylus is in its undeflected rest position, the two light beams produce a common light spot in the centre of the PSD. If the stylus is deflected, the two light beams each produce a light spot on the PSD, the two light spots migrating on the PSD depending on the size and the direction of the deflection. Depending on the position of the light spots, the PSD produces two output currents, with the aid of which the position of the light spots on the PSD can be determined. From the values obtained, it is then possible to determine the magnitude and direction of the stylus deflection. Owing to the particular type of arrangement of the light transmitters, deflections of the stylus in all three coordinate directions (x, y, z) can be detected in this case.
Another measuring head with optical detection of the stylus deflections is disclosed in WO 00/60307. In this case too, a photosensitive detector element measuring in two dimensions is used. The exact method of evaluation is, however, not stated in this publication.
DE 38 26 581 A1 discloses a measuring head which measures in three dimensions and in which a first optical detector element is used for the deflections of the stylus in the x or y direction and a second, spatially separate, optical detector element is used for detecting deflections in the z direction.
DE 27 41 413 A1 furthermore discloses a measuring head which measures in two dimensions and which the deflection of the stylus in the x and y directions is determined from the position of a light spot on a two-dimensional field of photocells. For this purpose, the detector field is divided into four quadrants and the position of the light spot in one or more of the quadrants is a measure of the deflection of the stylus.
The publication WO 2005/085749 discloses a measuring head comprising a stylus and its deflection in the lateral and vertical directions is read by producing a circular light image on at least two line sensors arranged spatially separately and offset relative to one another. The at least two, preferably four, line sensors forming a sensor cross are illuminated together and simultaneously by the one light figure. It is furthermore stated that line sensors are measuring elements for in each case only one dimension.
In the case of all measuring head systems described above and disclosed by the prior art, reading of planar sensors or a plurality of line sensors is absolutely essential, which proves to be very disadvantageous with regard to the speed of reading and hence also the speed of measurement.
In addition, there is a multiplicity of measuring heads in which the deflections of the stylus are detected not by an optical method but by other detectors, for example moving coils or hall elements.
The known measuring head systems each alone have individual strengths and weaknesses with regard to the maximum achievable accuracy of measurement, the maximum speed of measurement and the maximum possible measuring range and moreover with regard to size and weight of the measuring head, robustness and production cost. However, none of the known measuring head systems meets all requirements in the same way.